Typically an asshole that spends between five and ten minutes talking to you before mentioning casually that he/she is sick. Causing you to wonder if you've now been infected by what this mouth breathing fuck stick has. Usually its just the common cold, but it could just as easily be ebola.

I can't come in today, fuckin' Patient Zero there got me sick yesterday when he was breathing all over the coffee cups."

Patient Zero is the first human infected by a new or recently discovered viral or bacterial outbreak. They usually have the most basic, least mutated form of the disease, which may either make them invaluable to medical efforts or completely useless. They can also often be used to determine where the disease came from.

The 1st person infected by a zombie plague starting the zombie apocalypse/end of times. reffering to a mass infection of a populace either by a secret operation or misshap. This "zombie plague" swamps normal military and law enforcement organizations, leading to the panicked collapse of civilian society until only isolated pockets of survivors remain, scavenging for food and supplies in a world reduced to a pre-industrial hostile wilderness also known as Z day.

becky was the first infected with the virus...she's patient zero. lets kill that bitch.

see:Night of the living dead, zombieland, left for dead, 28 days later

The most effective way to contain the rise of the undead is to hit hard and hit often.